1. Field of the Invention
The present invention generally relates to semiconductor manufacturing equipment and more particularly to a wafer positioning system and method for positioning a semiconductor wafer.
2. Description of the Related Art
Semiconductor manufacturing equipment is used to process semiconductor wafers into electronic devices. Commonly, the equipment utilizes a number of different processing chambers accessible from a central wafer transfer chamber. Typically, the wafers are loaded into the processing system using a wafer carrier. A transfer mechanism, generally a robot, moves the individual wafers from the carrier, through valves, and into the various processing chambers. The robot may also move individual wafers between processing chambers to effect different processing steps. The movement of the wafers throughout the processing system is accomplished using automated handling techniques.
For reasons associated with manufacturing costs, processing times, and fabrication requirements, the volume of the overall processing system is kept to a minimum. Because of the limited volume, clearances and tolerances within the equipment system are necessarily minimized. In one such processing system, the clearance between a wafer and the walls of the equipment can be as little as a few millimeters. Unfortunately, with such limited space, wafers that become misaligned while being transported through the processing system may become dislodged, may be considerably damaged, and/or may cause damage to the processing equipment. As expected, breakage of wafers and/or damage to equipment is undesirable since each wafer and each component of the manufacturing equipment, represents a substantial investment to the manufacturer.
Typically, to precisely transport wafers through automated processing systems, wafer transport mechanisms or robots may be used. The robots are capable of performing precise and repeated tasks. However, the effectiveness of the wafer handling is dependent on the accuracy of the initial wafer position relative to the wafer transport robot.
Wafer processing systems may employ one or more techniques to precisely align the wafer transport robot and the wafer. In one technique, wafer positions are preferably detected by determining the position of the wafer transport robot when the leading and trailing edges of a wafer cross one or more optical beam paths during wafer transport. Position information derived from the wafer position data points may be compared with predetermined, nominal position information to determine the extent of the wafer misalignment. The misalignment information is preferably converted into a linear error component and a rotational error component. The linear extension and rotational displacement of the wafer transport robot is adjusted accordingly. In this example, the technique can be a relatively complex procedure, which usually leads to increased cost, increased equipment size, and increased processing time.
The present invention provides a system and associated method for aligning semiconductor wafers and wafer-like objects relative to a wafer transport mechanism. In the present invention, an image of a wafer is acquired using, for example, a CCD camera. The image is digitized and stored in a computer as an array (i.e. row and column) of pixels, each pixel representing a point on the image. The presence of a wafer against a contrasting background generates an intensity variation in the pixels of the acquired image. The discontinuity in the intensity in the pixels is typically associated with a boundary (e.g. an edge). Data points identified along the edge of the wafer are extracted and used to geometrically estimate the center of the wafer. As described in more detail below, the estimated wafer center is then compared to the position of a predetermined reference position to determine an offset. Using this information, the wafer transport mechanism can then be re-adjusted to pick up the wafer on the corrected center.
In the present invention, a single image of the wafer is needed, which allows for fast wafer positioning, while using minimal moving parts.
As described in more detail below, the single acquired wafer image can be used to orient the wafer, as needed, for processing. The present invention accomplishes the orientation task with less complexity, less cost and fewer sensors then most other techniques.
In one aspect of the present invention, a semiconductor wafer positioning system is provided. The system includes a transport mechanism, which is configured to transport a semiconductor wafer to and from one of a plurality of processing chambers. A position detection sensor provides an image from which a position of the wafer can be determined relative to a reference point.
In another aspect of the present invention, a method is provided for locating a position of a wafer. The method includes acquiring image data of a wafer for processing; processing the image data to determine the position of the center of the wafer; and moving a wafer transport assembly relative to the wafer to orient the wafer transport assembly relative to the center of the wafer.